Oregon Sea Grant

Quantification of contemporary sediment and carbon accumulation within Oregon tidal saline wetlands will: (1) fill a critical knowledge gap, and (2) naturally test without complicating variables whether sea level rise or sediment supply primarily control wetland growth. Here we measure vertical accretion rates and carbon burial rates in three Oregon estuaries – Youngs Bay, Tillamook Bay, and the Salmon River Estuary – of differing relative sea level rise rates (0.28 ± 0.44, 1.8 ± 0.4, and 2.2 ± 0.3 mm yr-1, respectively) and sediment loads (39, 160, and 24 t yr-1, respectively). Tillamook Bay and the Salmon River Estuary, which have similar rates of sea level rise but vastly different sediment loads, are accreting comparably (2.2 ± 0.3 and 2.4 ± 0.7 mm yr-1, respectively), indicating the importance of rising sea level and increased hydroperiod.

Youngs Bay, with the lowest rate of sea level rise and a moderate sediment supply, is accreting the quickest (2.7 ± 0.6 mm yr-1) suggesting the tidal wetlands are either not in steady state with sea level rise or suspended sediment may be higher in this estuary. Mean vertical accretion within the three estuaries is greater than sea level rise, all systems appear to have an excess of sediment, and Youngs Bay and Tillamook Bay exhibit rapidly accreting locations; therefore, despite many of these wetland’s inability to migrate landward, they will likely keep pace with projections of accelerated sea level rise over the coming century. Additionally, though the mean rates of carbon burial in these Oregon wetlands are about half the global average (94 ± 33 g Corg m-2 yr-1), carbon accretion is dictated by vertical accretion which will increase under imminent sea level rise.